The opening at the lower end of the bowel. It is kept closed by two muscles, the external and internal sphincters. The latter is a muscular ring which extends about 25 mm (1 inch) up the bowel, is nearly 6 mm (¼ inch) thick, and is kept constantly contracted by the action of a nerve centre in the spinal cord. In disease of the spinal cord the muscle may be paralysed, resulting in inability to retain the motions or stools.